This invention relates to particular zinc borosilicate glass compositions suitable for enamel coatings on soda-lime glass envelopes. More particularly, it relates to said glass compositions which are devoid of both lead and arsenic content in order to reduce any ecological hazards associated with these substances.
Lead and arsenic-free zinc borosilicate glass compositions are known and have been used as colored frits for bonding to both metal and ceramic substrates. For example, U.S. Pat. No. 3,005,722 discloses such a glass composition for use in ceramic type electroluminescent lamp devices wherein the uncolored frit serves as a binder of phosphor particles when fused to a porcelainized iron plate. Colored enamels using said type glass compositions are also disclosed in U.S. Pat. No. 3,527,649 which can employ various inorganic pigments such as cadmium, selenium, iron, copper, cobalt and other metal compounds to produce high colored glazes when fired on ceramic ware.
Other zinc borosilicate frit compositions generally containing lead and/or arsenic oxides have also been used as colored enamels for incandescent lamp envelopes. A typical known fit glass for adherence to the soda-lime glass bulb surface and which exhibits a high degree of weathering resistance has the following analysis in weight percent: ZnO--2.2, SiO.sub.2 --20.6, B.sub.2 O.sub.3 --14.8, Al.sub.2 O.sub.3 --2.2, Na.sub.2 O--1.8, K.sub.2 O--1.2, CaO--1.3, TiO.sub.2 --2.8, ZrO.sub.2 --6.2, F--0.3, PbO--44.4, CdO--2.2. Another known frit class exhibiting less weathering resistance but which has been found satisfactory for indoor use type lamps has a typical analysis in weight percent as follows: ZnO--28.3, SiO.sub.2 --19.5, B.sub.2 O.sub.3 --21.7, Al.sub.2 O.sub.3 --2.7, Na.sub.2 O--4.9, K.sub.2 O--6.1, CaO--4.3, BaO--3.7, TiO.sub.2 --4.5, F--2.7, Sb.sub.2 O.sub.3 --1.6. Both of said glass frit compositions exhibit a linear thermal coefficient of expansion in the 0.degree.-300.degree. C. temperature range of approximately 70-80.times.10.sup.-7 /.degree. C. in order to match the thermal expansion characteristics of the soda-lime glass envelope when bonded thereto at the elevated firing temperatures. The lead oxide content in the first of said frit glass compositions provides greater water insolubility, hence is responsible for the higher degree of durability or weathering resistance found.
Because of the recognized objections to having either lead oxide or arsenic oxide as a constituent in surface coatings, however, it would be desirable to provide a frit glass devoid of said substances but which can exhibit equivalent weathering durability. On the other hand, any removal of lead oxide from this type zinc borosilicate glass composition results in a zinc borate containing glass which has significant water-solubility that can lead to dulling of the enamel surface when exposed to outside weather conditions. Accordingly, some further adjustment of this type frit glass composition is required in order to produce a satisfactory weather-proof enamel that can be applied in the customary manner to a soda-lime glass envelope.